Cities are getting warmer. This is due in part to global climate change. The more important factor for now, though, is the urban heat island effect, local warming in cities caused by sidewalks, asphalt, and reduced tree cover. In short, areas with less shade and more impervious surface area absorb more heat and trap it – releasing it more slowly than other areas after the sun goes down. These urban heat islands have a number of consequences for trees. Trees planted in warmer areas within cities often have more insect pests, and these same trees grow less than trees in cooler areas nearby.

As part of my graduate thesis, I studied how urban warming and insect pests affect street trees, and a mystery began to nag at me. Some trees in the hottest areas within cities were covered in insect pests and still looked vigorous, while other trees with the same pest densities withered. I thought this pattern might be because trees have access to different amounts of resources—depending on where and how they are planted—and that trees with more resources might be better able to tolerate or compensate for insect pests. This is a simple idea that had not been tested, and its implications are of critical importance. People are healthier when we have access to greenery, and urban trees can only provide ecosystem services – such as reducing air pollution and trapping carbon — if they can photosynthesize and grow.

To approach this mystery, I did an experiment across the city of Raleigh, NC and another, more controlled experiment in the laboratory. In the city, I chose forty trees across a gradient of urban temperatures. At each site, one tree was sprayed for pest insects, and another was left unsprayed. In the lab, I did essentially the same experiment but also manipulated the amount of water trees had access to. Water is often scarce for urban trees because they have limited rooting space and are often planted in compacted soils.

I found that trees in hotter areas of the city were more water stressed. In the lab, water stress reduced tree growth, but warming and insect pests acting alone did not affect tree growth. However, trees that were water stressed, hot, and infested with insect pests grew the least. What this means is that trees can compensate for insect pests and warming if they have enough water. If trees don’t have enough water, warming and insect pests combine with water stress to reduce tree growth. Together, these experiments showed that some urban trees may be more affected by pests than others because they have access to less water.

Street trees in the hottest areas of the city were more water stressed. Photo: EK Meineke

These findings provide information on how we might be able to better manage urban forests so that cities are green and vibrant in the future:

1) Revisit how we assess insect pests. To keep cities green, entomologists and land managers periodically count insect pests on plants. However, counting insects might not be the best way to understand how pests affect plants, because a tree’s ability to make up for what insects remove depends on the resources they have access to. Urban forests may benefit from a more holistic process for assessing when insects are actually acting as pests and, on the other hand, when we expect that trees are able to compensate for their damage. This framework could reduce how much we have to treat trees with pesticides, which are major threats to biodiversity.

2) Make sure trees have enough water. As trees get hotter due to climate change and urbanization, the amount of water they have access to will become even more important for tree growth than it is now. Our results suggest that, for many trees, providing adequate rooting space and watering during heat waves may be key strategies for reducing the effects of warming, water stress, and insect pests as they intensify with local and global warming.

3) Plant for the future. When planting trees in cities, we may want to consider drought-resistant plants and genotypes to maximize the number of large shade trees in future urban forests. One area of future research is to determine if these drought-resistant plants are also better able to compensate for insect pests.

Planting flowers is a good start, but it’s a bit more complicated. Photo: April Hamblin

If you build it they will come. Local politicians pilfer this line when coaxing constituents to support development projects. Build the waterfront, tourists will come. Build the stadium, jobs will come. Conservation types also administer this line when promoting habitat protections or improvements to help imperiled species. Threadbare as it is, the sentence still generally applies since species cannot exist without habitat except in zoos. Build new habitat, protect or improve old habitat, and your species of interest might arrive.

Pollinator enthusiasts have ardently embraced this idea. There has been an explosion of pollinator conservation activities such as using pollinator friendly plant species and gardening practices, installing pollinator gardens, and providing nesting resources. Again, if you build it – provide the necessary resources – the bees will come and their populations may thrive. This makes sense and is a worthwhile practice just like installing bluebird nest boxes.

But what if you built nest boxes in a wind farm or highway median? The birds would probably come, but obvious hazards may prevent them from thriving. Our research on native pollinators suggests this situation exists for bees in cities. Urban yards can be tough for bees. There are often not enough flowers, or the wrong kinds of flowers, so people compensate with pollinator gardens. However, cities are also hot, due to impervious surfaces and the urban heat island effect. Wild bees differ in how well they tolerate high temperatures, so heat-sensitive species like Agapostemon virescens and Megachile mendica are less abundant in hot urban sites.

Agapostemon virescens. Photo: Sam Droege, USGS

Megachile mendica. Photo: Sam Droege, USGS

So, can planting flowers compensate for the negative effects of urban heat? We studied wild bees in 18 urban yards and parks throughout Raleigh, NC, that had different temperatures, area of impervious surface cover, and density and diversity of flowers. In a new paper by April Hamblin and Elsa Youngsteadt, we describe how, at the hottest sites, bee abundance declined regardless of flower abundance or diversity.

Although large species of bees (such as bumble bees or sunflower bees) did become more abundant with increasing flower density, small bees (such as sweat bees) suffered the negative effects of warming with no counteracting benefit of flowers. In fact, total bee abundance declined 41% for each degree Celsius of warming. Thus, in a yard with an average temperature of 25°C we collected around 300 bees. In a hotter yard nearby with an average temperature 27°C we collected just 100 bees. In addition, the composition of species was different, with fewer heat-sensitive species at hot sites and more heat-tolerant species.

We used vane traps (above) and bee bowls (right) to examine bee abundance and diversity in the field. Photo: April Hamblin

Photo: April Hamblin

Wild bee abundance also declined in areas with more impervious surfaces like roads and sidewalks, which are also bad for honey bees. This makes sense because impervious surfaces make the air hotter and because any area covered by concrete is not available for nesting or planting flowers. A double whammy.

Large bees, such as this bumblebee, did become more abundant when more flowers were present. Photo: Elsa Youngsteadt

The good news? Our research confirms that if you plant flowers – if you build it – bees will come. Bee abundance and diversity increased with flower abundance even though temperature and impervious surface mitigated the number and types of bees. Thus, in addition to planting flowers, try to reduce the amount of heat and impervious surface around your yard or park. Of course you can’t tear up sidewalks and roads (please don’t) but planting trees to shade those surfaces will help cool things down. There’s more to building bee habitat than planting flowers.

This study was funded in part by an Agriculture and Food Research Initiative Competitive Grant (2013-02476) from the USDA National Institute of Food and Agriculture to S.D. Frank and E. Youngsteadt. This work was also funded by Cooperative Agreement No. G11 AC20471, G13 AC00405, and G15AP00153 from the United States Geological Survey to S.D. Frank.
]]>Silk tents and spacesuitshttp://ecoipm.org/2018/01/25/silk-tents-and-spacesuits/
Thu, 25 Jan 2018 14:40:33 +0000http://ecoipm.org/?p=7388

Arriving at the office. Photo: Annemarie Nagle

This post was co-written by Frank Lab manager Annemarie Nagle and PhD student Kristi Backe, who is currently studying the pine processionary moth in France. Annemarie recently traveled to France to help Kristi install temperature loggers at field sites around the country. Read more about the project HERE.

It’s winter in the northern hemisphere, and for many of us, that means piling on lots of layers, cranking up the thermostat, and microwaving the water in our water bottle before we drink it. (Okay, maybe that last one is just for extreme cold-haters like Annemarie, but you get the point.) We’re all taking measures to stay toasty this time of year.

But what do our ectothermic friends do when the temperature falls? Many insects deal with winter by riding it out as pupae or eggs, “resting” life stages in which the insect doesn’t move around or eat. The pine processionary moth (PPM; Thaumetopoea pityocampa), on the other hand, spends winter as a caterpillar that must stay warm enough to feed and grow during the cold months.

Tent caterpillar nests are a common sight along roadsides in the U.S. In large parts of Europe, the most noticeable nests in trees are made by the caterpillars of the pine processionary moth. Photo: Kristi Backe

PPM caterpillars live in big groups (“colonies”) inside silk tents that they build in pine trees. The caterpillars hang out in these nests during the day, only emerging at night to feed on the tree’s needles. As temperatures get colder, the caterpillars add more layers of silk to the tent, eventually making a thick, tough shelter that is protection not only from predators but also from the elements. Generally, the caterpillars will build their tent on the edge of the branches in the sunniest part of the tree to maximize the sun’s warming effects. This turns out to be an effective strategy, and it can be up to 20°C warmer inside the tent on a sunny day than it is outside.

Having a cozy tent is important for the obvious reason that freezing to death is bad, and at the northern part of the PPM’s range, near Paris, freezing to death is a real possibility. But the nest is also important because the caterpillars have very specific temperature thresholds that determine if they can feed. They can only feed on nights where (1) nighttime air temperatures are above freezing (0°C) and (2) the caterpillars experienced temperatures above 6°C during the previous day. This 6°C mark is called the “activation temperature,” and if the caterpillars haven’t been activated, they won’t feed at night even if the nighttime temperature is above freezing. Thanks to their sun-exposed tent, the caterpillars often experience daytime temperatures above the activation threshold even when the outdoor temperature is too cold. In other words, because their nest is super warm, the caterpillars are able to feed on nights when they otherwise couldn’t.

Caterpillars might also get a boost by living in cities. Cities are typically warmer than surrounding forests, and this could mean that city-dwelling caterpillars reach the activation temperature more often. At night, when the temperature difference between cities and forests is the greatest, caterpillars in urban areas might spend more time above 0°C, giving them extra time to feed. With more opportunities for feeding, city caterpillars may grow faster and be larger, making them more resilient to the really cold temperatures that come along later in the winter.

In order to test hypotheses about how PPM caterpillars in the city compare to ones in forests, it’s important to understand exactly what temperatures the caterpillars are experiencing. This means we need a temperature data logger inside the nests.

On the surface, this doesn’t sound like an onerous task, but these caterpillars are pretty pesty. Each larva is covered in patches of barbed, skin-irritating hairs that they can shoot off at any moment if disturbed. So unless you want an itchy rash, your best bet is to steer clear of these little critters altogether. And if you’re a couple of scientists trying to insert a data logger into a nest filled with two hundred of them? Well, your best bet is to suit up in some protective gear.

We hung temperature loggers outside the nests too, but that was the easy part. Photo: Alain Roques

A PPM nest after the caterpillars have sealed the temperature logger inside. You can just make out a corner of the yellow plastic rectangle that holds the logger in the nest. Photo: Alain Roques

Fortunately, it’s usually pretty easy to insert the data logger without majorly disturbing the caterpillars, and they get over it pretty quickly. Within a day, they’ve already patched up the hole in the nest with more silk (sealing the logger inside), and they go about their business. And we moved on to the next field site, taking in the beautiful French countryside along the way.

This is the second in a series of posts about Kristi’s pine processionary moth research. (See the first one HERE.) At the end of the winter, she’ll head back to all of the sites to dig out the loggers and see what the caterpillars have been doing. Stay tuned for an update. Kristi’s project is supported by NSF GROW and a Chateaubriand Fellowship from the Embassy of France in the U.S.
]]>Do itchy caterpillars have an advantage in cities?http://ecoipm.org/2018/01/17/do-itchy-caterpillars-have-an-advantage-in-cities/
Wed, 17 Jan 2018 16:16:33 +0000http://ecoipm.org/?p=7353This is a guest post by PhD student Kristi Backe, who is currently studying the pine processionary moth in France.

The caterpillars of the pine processionary moth (a.k.a. “PPM”) shoot itchy, projectile hairs that cause allergic reactions in humans, pets, and other animals. They also work in groups to build big, ugly silk nests in pines and other coniferous trees. Obviously, this doesn’t make PPM a fan favorite, but that hasn’t stopped entomologists from studying it intensively for decades (in fact, it’s mostly why).

The PPM is native to the Mediterranean Basin, and as winters have become warmer over the last several decades, it’s been expanding its range farther north. For many years, scientists have used forest maps to focus their predictions about where the PPM might spread. The underlying idea, and it’s a good one, is that the PPM isn’t going to survive for too long in areas with no food. But, it’s becoming increasingly clear that the PPM doesn’t actually need forests. It just needs trees, and it’s willing to accept the ones in the front yard, thank you very much.

In 2012, a group of French scientists led by Christelle Robinet found evidence that the PPM might actually be doing better on trees in cities than on trees in forests because cities are warmer (a phenomenon scientists have named “the urban heat island effect”). If you’ve been following this blog for awhile, this idea probably sounds familiar. (And if you haven’t, check out this, this, and/or this.) Because I study the distribution of insects in urban areas and I want to spend eight months of my life eating cheese and baguettes, I jumped at the chance to head to France to study how the PPM is doing in cities.*

At the end of August, I started a stint with Alain Roques in the Forest Zoology lab at the French National Institute for Agronomic Research (INRA) in Orléans, France, to try to figure out whether the warmer temperatures in cities are giving urban-dwelling PPM caterpillars an advantage. The end of the summer might seem like a strange time to start an entomology field season, but the PPM caterpillars buck the usual trend and are active in the winter. They also feed at night, when the urban heat island effect is the strongest, which (fingers crossed) makes them a good study subject for this type of research.

A black pine (Pinus nigra) nursery infested with PPM. The big white “ornaments” on the two middle trees are PPM nests. Each nest contains hundreds of caterpillars. Photo: Kristi Backe

Field work in France is equal parts caterpillars and castles. Photo: Kristi Backe

In general, there are two main ways that warmer temperatures in cities could help the PPM survive during winter at the northern part of its range: 1) if caterpillars grow faster in the city’s warmer temperatures, they may be bigger and hardier than the forest caterpillars when cold temperatures finally arrive, and 2) if the minimum temperatures during a cold snap aren’t quite as cold in the city as in the forest, it’s less likely that caterpillars will freeze to death. Now all that’s left is figuring out whether either of these is making a difference for the PPM in urban areas.

I’ll be following up with a couple more posts about how we’re trying to answer our research questions and what it’s like to spend my days with millions of allergenic insect hairs.** In the meantime, enjoy this video of young PPM caterpillars doing their thing:

* If, like me, you’re a PhD student interested in overindulging in cheese and baguettes during your dissertation work, check out the Chateaubriand Fellowship Program from the Embassy of France in the U.S. Students with NSF graduate research fellowships (MS and PhD) can also apply for supplemental funding through NSF GROW. Thanks to both of these programs for supporting my trip!

** Remember when you were little and you dreamed of having a big-kid job where you’d be pelted in the face by barbed, projectile caterpillar hairs? Me either.

The Museum of Natural Sciences held another magnificent BugFest, with the dragonfly as the main feature of 2017. This annual event brings 35,000 insect enthusiasts to downtown Raleigh, where more than 100 stations with arthropod related displays, educational activities, and specimens are set up. As usual, the Frank Lab manned a table, this year with the theme of “Your Backyard Jungle: Meet the Six-legged Neighbors.”

While the stars of last year’s table were undoubtedly the azalea caterpillars—which were very resilient during a long day of being picked up, and dropped, by visitors of all ages—the azalea bushes were empty when it came time to collect this year. We think that azalea caterpillars may have squeezed in a second generation at the end of summer in 2016, while in 2017, some late-summer storms made for (relatively!) cooler days in North Carolina.

Instead, the six-legged neighbors of 2017 were represented by wheel bugs (also known as assassin bugs), a male and female praying mantis, pine sawfly larvae, katydids, a beautiful yellow garden spider (also known as a writing spider), and lots of caterpillars: swallowtails, silver-spotted skippers, a fat tomato hornworm, and this year’s favorite—the curve-lined owlet moth caterpillars which look just like dried leaves dangling and trembling in a breeze. Kids and adults enjoyed asking questions about these common insects that are easily collected in a backyard or park.

Frank Lab members Samantha, Ian, and Larry talk to booth visitors at Bugfest.

Nora’s 5-year-old son poses for a 6-legged portrait

With Steve on sabbatical, Kristi studying in France, and Elsa at the Predatory Plants table, this year’s Backyard Jungle was organized by graduate student Christina Mitchell and manned by almost the entire Frank Lab over the course of the day. Later that evening, the stars of the show were released by an extremely excited 5-year-old, back where they belonged…even the tomato hornworm.

]]>News from Oregonhttp://ecoipm.org/2017/10/02/news-from-oregon/
Mon, 02 Oct 2017 20:37:26 +0000http://ecoipm.org/?p=7236I will be in Eugene, Oregon for the next few months. Thanks so much to the great folks at University of Oregon Institute of Ecology and Evolution for hosting me and lending me office space. I’m particularly grateful to Nadia Singh, who just arrived here herself, for sharing her office with me (and the team of children they keep on staff to make signs). I give a seminar for the department October 3.

I have also been working in less formal offices, such as this spot in Hendricks Park, looking at trees, bees, and other things and trying to learn more about them. I will continue posting about pests, lab news, and cool (to me) tidbits about natural history and ecology. Stay tuned.

View from the office at Hendricks. Photo: Steve Frank

]]>Pests of the past re-emerge on elmhttp://ecoipm.org/2017/09/19/pests-of-the-past-re-emerge-on-elm/
Tue, 19 Sep 2017 18:15:13 +0000http://ecoipm.org/?p=7158Elm trees had all but disappeared from US streets after Dutch elm disease swept across the continent during the 20th century. Now, many Dutch elm disease resistant or tolerant cultivars are available, having undergone years of development and evaluation. Urban foresters are always thirsty for new trees that perform reasonably well along streets and in other urban settings. So now these new elms are being planted in enormous numbers across the country.

It remains to be seen how well these elms hold up as they are susceptible to many pests and structural problems. The wooly elm aphid, Eriosoma americanum, is one of those pests. Besides basic life history, this creature it is essentially unstudied. However, I found a cool publication from 1913 and one from 1910 that describe the appearance and natural history of this and other aphids on elm. (Both of these publications are by Edith Marion Patch, who deserves a whole post or book of her own. She was a scientist and writer who organized the department of entomology at University of Maine in 1901 and was its head for 33 years).

I am in Eugene, Oregon and saw wooly elm aphids congregating on elm trunks while ants were collecting and eating them. Wooly elm aphids each lay a single egg in a nook or cranny of elm bark in late summer or early fall. In spring those eggs hatch and the aphids climb up to feed on elm leaves and mature. These mature aphids produce a second generation asexually that mature into winged females. The female aphids fly to Amelanchier trees where they produce another asexual generation on the leaves. After one generation on leaves the aphids crawl down and feed for multiple generations on Amelanchier roots forming dense colonies.

Finally, a winged generation is produced that flies back to elm to produce a sexual generation of males and females. These females oviposit and die relying on their lone egg to overwinter. The aphids I am seeing now in Eugene are probably the females trying to lay their one egg. Unfortunately for them, there are other phloem-feeding insects in the tree attracting ants, which are grabbing aphids as they head up and down the trunk.

Wooly elm aphids on an elm trunk. Photo: S.D. Frank

Ants eating wooly elm aphids on an elm trunk. Photo: S.D. Frank

Damage by wooly elm aphids on elm is usually not severe. Leaves become curled from the edges inward and contain hundreds of aphids along with wooly wax. However, as I mentioned, there are many other elm pests including several other aphids described in books by Edith Marion Patch and fact sheets such as this one from the Ohio State University.

]]>A Day in the Life of an Urban Ecologisthttp://ecoipm.org/2017/07/13/a-day-in-the-life-of-an-urban-ecologist/
Thu, 13 Jul 2017 13:30:58 +0000http://ecoipm.org/?p=7086This is a guest post from PhD graduate student Sarah Parsons

I hunch over a freshly cut tree twig and I count: 1…2…3… It is a breezy day, for which I am grateful. Counting insects on city street trees can be a warm endeavor, a phenomenon we can attribute to the “urban heat island” effect. It is often a few degrees hotter in urban sites when compared to surrounding suburban and rural areas. I count to 18 and throw the twig in the back of the truck. Today I am counting crape myrtle aphids, Tinocallis kahawaluokalani. In fact, almost every field day of my urban ecologist career I have counted these little critters, who occupy a space not much larger than a pen head.

Crape Myrtle Aphids go through many life stages (instars) before reaching their winged adult form. Here you see second and third instar nymphs and a winged adult.

I make another clipping and repeat my counting: 1…2…3… This time someone startles me and I have to stop my counting. A woman comes walking briskly across an adjacent lawn. Her eyes are set on me.

I can expect this encounter to go one of two ways. This is a woman with an insatiable appetite for learning, whose scholastic intrigue cannot keep her from asking me what I am doing, OR she is a slightly disgruntled neighbor. She is the latter. When she approaches, I calmly explain who I am, a researcher at NC State University, and what I am doing, counting crape myrtle aphids. I tell her that her neighborhood tree won the prize for being the ONE tree picked out of thousands in the city database to be sampled. This last line usually makes people laugh. This particular neighbor was not amused. I then explain that this is a city street tree, on which we have permission to sample. I also explain that I make 4 small clippings from the tree to count aphids. I show her the clippings, and her body language relaxes. My explanation has been successful. I have established that no harm to the tree is being done, and that I am nothing more than a lowly graduate student trying to pave my path toward a publication.

In order to get a good sampling of crape myrtle aphids on a tree, it is good practice to count aphids on several areas of the tree canopy. Here you see me taking a clipping from a lower area of the canopy. During each tree visit I take four clippings from both high and low areas of the canopy.

Although this woman’s approach was not atypical, it is not common. Most people who approach me while sampling trees are intrigued. They want to learn about my aphids and their crape myrtles. The conversation usually segues into one about when they should prune their trees or how to treat for other pests in the garden.

I have been asked in for tea and chocolate, have heard life stories, and have been offered jobs to do landscaping. This is the norm. This is being an urban ecologist. I enjoy this part of my job, and I find it to be the part of my job that sets my field apart from others. Many ecologists work in settings secluded from humans, but in urban ecology humans are a part of the landscape and the ecosystem. In fact, humans are integral in the ecosystem, especially if you are studying a tree like crape myrtles, which would not exist in North Carolina without human intervention.

In my small corner of urban ecology I study the landscapes around crape myrtle street trees to ask larger questions about how we can attract beneficial predatory insects and therefore reduce pests on crape myrtles, such as the crape myrtle aphid. I hope my research will inform future landscape design around street trees and reduce overall pesticide use on city trees.

After my encounter with the neighbor I make another clipping and start counting again…1…2…3… The slight breeze dries the sweat on my brow and makes snow of the crape myrtle flowers, which rain down on my sun hat. I smile. Just another day in the life of an aspiring urban ecologist, I think to myself. I finish counting, throw the clipping in the truck and head off down the road to my next adventure.

Pink-striped oakworms are chomping on oak trees in Raleigh right now and leaving piles of frass (poop) in their wake. They’re not too choosy about oak species, and this week I found them on sawtooth oak, white oak, and pin oak. Spot them on your trees by looking for branches with all of their leaves chewed off.

Pink-striped oakworm, Photo: KM Backe

You might be more familiar with the orange-striped oakworm, a closely related species that will be out in full force in a month or so. We already have a few posts about orange-striped oakworms up on the blog (HERE, HERE, and HERE), and the management tips in those posts will work for pink-striped oakworms too. Bottom line: oakworms don’t usually do serious damage, and the easiest way to get rid of them (if you bother to get rid of them at all) is to prune off the branch that they’re feeding on. They feed in large groups, so you can wipe out a lot of them with one cut.

Pink-striped oakworms feed in groups. Photo: Kristi Backe

Typical defoliation on an oak branch. Photo: Kristi Backe

You can also count on predators and parasitoids to help keep the oakworms under control. This week I noticed a Tachinid fly laying eggs on a bunch of caterpillars.

Can you spot the Tachinid fly? Photo: Kristi Backe

She would lay an egg and then move on to the next caterpillar in the line. The fly larvae that hatch from those eggs will eat the caterpillars from the inside out – a little gory, but it will help keep the oakworm populations down!

I walk a lot. It hurts less than running and is more fun than swimming but still has some minimal health benefit. Sometimes I walk for miles. I listen to some NPR podcast and think about my route. I choose where to turn and the route I take based on two main criteria: I like to conserve potential energy by picturing the contour map of my neighborhood in my head and try not to go down hill just to needlessly go back up (like I said, minimal health benefits). Second, I don’t like to be in the sun so I think about where there are trees and shade, even if it means going down hill.

Sunlight and temperature vary block by block and sometimes house by house. This is due to the shade that trees provide and the amount of pavement that absorbs heat from the sun and releases it back into the environment. In a matter of blocks we have found that air temperature can change by several degrees C. Imagine if your air conditioner was set at 86 F (30C) instead of 77 F (25C). Lethal? No. Noticeable to a sensitive reader of science blogs? Yes.

So what about the bees already?! Insects experience the environment at much smaller scales than people do. If I’m hot I can look ahead see a shady block and walk there (hopefully down hill). Some insects may not move more than a block in their lives. If they are born in a hot block they may be stuck in a hot block or if not stuck they may have to spend a lot of time and energy to escape. So we figured bees that can’t tolerate heat won’t be found in hot parts of town.

Xylocopa virginica(carpenter bee) was one of the native bee species included in the study. Photo: Elsa Youngsteadt

To test this hypothesis we collected 15 species of wild bees around Raleigh, NC and challenged them to see how high a temperature they could stand. We put them in test tubes and heated the tubes slowly until they could not walk or fly. Unpleasant as it was, this procedure tells us, at a physiological level, which bees are more tolerant of heat than others. Some bees fizzled out at 45 C but others lasted until 51 C. That’s a big difference.

Once we knew the range of temperatures bees could tolerate we sampled yards around Raleigh that varied from very urban and hot to less urban and cooler to see where our study bees lived. We found 18 yards and parks that we sampled 11 times over 2 years. We counted and identified a lot of bees then compared the range of yard temperatures to the range of bee heat tolerance.

It turns out that bees with the lowest heat tolerance in the lab were least abundant in the hottest yards. Bees with the highest heat tolerance in laboratory experiments managed to hold their own, even in hot yards.

This may seem obvious. Hot bees can handle hot yards, cool bees need cool yards. However, there are a couple reasons this work is important. First, no bees were more abundant in hot yards. Some bees just manage to hang on in the hot yards. This means that as cities (and the whole world) gets hotter, all the bees we studied will do worse.

Second, to the extent that some bee species avoid hot areas, all the flowers and ‘pollinator gardens’ in the world won’t help the species that get too hot and die. Bee species that get too hot in NC will have to migrate north to find more hospitable temperatures. If they move north it is one less species here. If that species pollinates some important crop or some endangered plant, that plant is out of luck.

When a habitat gets hotter due to urban heat or global warming some species will fizzle out. Our experiments show that we can predict which species will thrive (or at least hang on) in hot habitats and which species will disappear. Wild pollinators are struggling amid multiple stresses. Hopefully work like ours can help predict which pollinator species (and plants that depend on them) will hang on and which may need extra help.